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Spinor Exciton Condensation in Coupled Quantum Wells

耦合量子阱中的自旋激子凝聚

基本信息

批准号：
EP/H032258/1
负责人：
Marzena Szymanska
金额：
$ 12.27万
依托单位：
University of Warwick
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2010
资助国家：
英国
起止时间：
2010 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FH032258%2F1
关键词：
Spinor Exciton Condensation Coupled Quantum

项目摘要

The search for new quantum phenomena and new quantum states of matter is currently a highly active area of experimental and theoretical investigations. The aims are twofold: to understand fundamental properties of matter and the extremes to which we can push it, and to use them for practical applications. The aim of this project is to explore the properties, and to help in experimental realisation, of a novel quantum state in semiconductors, called: spinor exciton condensate.One of the most exciting macroscopic quantum states, sometimes called ``the fifth state of matter'', is a Bose-Einstein condensate (BEC). An important property of condensates is quantum coherence -- a special type of order which also underlines the unique properties of laser light, superconductors and superfluids flowing without resistance. The first realisation of BEC took place in 1995 in a gas of rubidium atoms cooled to nano-Kelvin temperatures. The idea of BEC in semiconductor electron-hole systems, triggered by the formulation of the BCS theory, dates back to the early days of research on BEC and superconductivity. It has been discussed, that the BCS-BEC state can be created in semiconductors by external excitations of electrons, leading to the formation of electron-hole bound states -- the excitons -- solid state analog of hydrogen. Due to the light effective mass of excitons, excitonic BEC is expected to take place at temperatures of the order of kelvins, i.e. orders of magnitude higher than that for atomic alkali gases, bringing hope for practical device applications. However, BEC has the chance to form only if the excitonic recombination rate is sufficiently slow, i.e. slower than the thermalisation and condensate formation rates, and if sufficiently large densities of excitons can be achieved within their lifetime. This has proven to be the major technical obstacle in the realisation of excitonic BEC, and almost half a century after the theoretical proposal the experimental evidence of this state remains unconvincing. However, due to the large technological progress in the sample growth and effective exciton trapping in recent years it is expected that, following the example of microcavity polariton BEC undergoing a real blossoming in the last three years, the exciton BEC should be within experimental reach. In this context, over the last decade coupled quantum wells have emerged as a promising system to achieve Bose condensation of excitons, with numerous experimental studies aimed at the demonstration of this effect. Since the electron and hole wavefunctions in the two wells have little overlap, excitons in this type of structure have much longer lifetimes. Further, by applying mechanical stress one can create effective exciton traps, and thus densities sufficient for BEC.In coupled quantum wells under stress the physics is quite complex: strain induced coupling, spin-orbit and piezoelectric effects lead to mixing of various exciton spin states. Thus, we should expect a spinor bright-dark condensate in these structures. The ultimate goal is to realise and study exciton BEC. However, in order to achieve this goal it would help if basic fundamental questions concerning excitons in coupled quantum wells under strain were understood: (i) what is the nature (spin structure) of the ground state? (ii) what are the scattering properties of excitons with the spinor structure and dipolar interactions? (iii) how are the many-body features, and signatures of BEC and superfluidity, affected by this structure and scattering? By effectively combining our theoretical analysis and experimental investigations of our academic collaborators, our project will address these questions. Quantum condensates have already found applications in high precision measurement, atomic clocks and inertial sensors of unprecedented accuracy. With the recent realisation of these unique states in the solid state, there is definitely more to come.

寻找物质的新量子现象和新量子态是当前非常活跃的实验和理论研究领域。目标是双重的：了解物质的基本性质和我们可以将其推向的极端，并将其用于实际应用。这个项目的目的是探索半导体中一种新的量子态的性质，并帮助实验实现：旋量激子凝聚态。最令人兴奋的宏观量子态之一，有时被称为“物质的第五态”，是玻色-爱因斯坦凝聚态(BEC)。凝聚体的一个重要性质是量子相干性--这是一种特殊的有序类型，它也突显了激光、超导体和超流体无阻力流动的独特性质。第一次实现BEC是在1995年，在一种冷却到纳米开尔文温度的Rb原子气体中。BCS理论的提出引发了半导体电子-空穴系统中BEC的概念，这可以追溯到BEC和超导电性研究的早期。讨论了半导体中的BCS-BEC态可以通过电子的外部激发而产生，从而形成电子-空穴束缚态--激子--类似于氢的固体状态。由于激子的光有效质量，激子BEC有望在开尔文数量级的温度下发生，即比原子碱性气体的温度高一个数量级，这为实际器件应用带来了希望。然而，BEC只有在激子复合速率足够慢的情况下才有机会形成，即比热正化和凝聚体形成速率慢，并且如果激子密度足够大，在它们的寿命内。这已被证明是实现激子BEC的主要技术障碍，在理论提出近半个世纪后，这种状态的实验证据仍然不令人信服。然而，由于近年来在样品生长和有效的激子捕获方面取得了很大的技术进步，预计在最近三年微腔极化BEC经历真正蓬勃发展的例子之后，激子BEC应该在实验范围内。在此背景下，在过去的十年中，耦合量子阱已经成为一种很有前途的实现激子玻色凝聚的系统，大量的实验研究旨在证明这一效应。由于这两种结构中的电子和空穴波函数几乎没有重叠，这种结构中的激子具有更长的寿命。此外，通过施加机械应力，人们可以产生有效的激子陷阱，从而产生足够的密度来满足BEC。在应力作用下的耦合量子井中，物理是相当复杂的：应变诱导耦合、自旋-轨道和压电效应导致各种激子自旋态的混合。因此，我们应该期待在这些结构中有一个自旋的明暗凝聚体。最终目的是认识和研究激子BEC。然而，为了实现这一目标，如果了解关于应变下耦合量子阱中激子的基本基本问题将是有帮助的：(I)基态的性质(自旋结构)是什么？(Ii)具有旋量结构和偶极相互作用的激子的散射特性是什么？(Iii)这种结构和散射对BEC和超流的多体特征和特征有何影响？通过有效地将我们的理论分析和对我们的学术合作者的实验调查相结合，我们的项目将解决这些问题。量子凝聚体已经在高精度测量、原子钟和具有前所未有精度的惯性传感器中得到了应用。随着最近对固态中这些独特状态的认识，肯定还会有更多的事情发生。